In all imaging systems utilizing visible light, optics is an important tool to increase the performance for the imaging task. The optics can for example enable higher spatial resolution through magnification and also higher fluxes by collecting the light rays.
In X-ray imaging this is not true, in e.g., medical x-ray imaging, there are no x-ray optics in regular clinical use. The explanation is that for energies exceeding around 15 keV, the difference in refraction index in any material compared to vacuum is very small, several orders of magnitude smaller than for visible light. This means that any optics are very hard to construct. At lower X-ray energies, so called zone plates are successfully used in many applications, while at higher energies they become increasingly inefficient and difficult to manufacture. In spite of these challenges, some X-ray optics have been tested to also work at higher energies. One example is grazing incidence optics as described in U.S. Pat. No. 6,949,748 where the x-rays hit a curved surface at a very small angle. Other examples are refractive optics as outlined in U.S. Pat. Nos. 6,668,040 and 6,091,798 and also the so-called phase array lens as described in B. Cederström, C. Ribbing and M. Lundqvist, “Generalized prism-array lenses for hard X-rays”, J. Sync. Rad, vol 12(3), pp. 340-344, 2005.
A summary of state of the art x-ray optics can be found in “Soft X-Rays and Extreme Ultraviolet Radiation—Principles and Applications”, David Attwood ISBN-13: 9780521029971, Cambridge University Press 2007. The optics for higher energies are generally one dimensional which sometimes fits the application, such as imaging using scanning line detectors, but in most cases optics that work in two dimensions is desirable. This can be achieved by crossing two one dimensional lenses, putting one after the other. This however results in a bulky device with compromised performance since absorption is increased and the two dimensional performance becomes sub-optimum by using one dimensional devices. This may be why these arrangements are not in wide practical use, or in fact, are hardly used at all for any application.